1. Field of the Invention
This invention relates generally to semiconductor device manufacturing, and, more particularly, to a method and apparatus for controlling the flow of wafers through a process flow.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a widely used means of planarizing silicon dioxide as well as other types of layers on semiconductor wafers. Chemical mechanical polishing typically utilizes an abrasive slurry disbursed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action. Generally, a chemical mechanical polishing tool includes a polishing device positioned above a rotatable circular platen or table on which a polishing pad is mounted. The polishing device may include one or more rotating carrier heads to which wafers may be secured, typically through the use of vacuum pressure. In use, the platen may be rotated and an abrasive slurry may be disbursed onto the polishing pad. Once the slurry has been applied to the polishing pad, a downward force may be applied to each rotating carrier head to press the attached wafer against the polishing pad. As the wafer is pressed against the polishing pad, the surface of the wafer is mechanically and chemically polished.
As semiconductor devices are scaled down, the importance of chemical mechanical polishing to the fabrication process increases. In particular, it becomes increasingly important to control and minimize within-wafer topography variations. For example, in one embodiment, to minimize spatial variations in downstream photolithography and etch processes, it is necessary for the oxide thickness of a wafer to be as uniform as possible (i.e., it is desirable for the surface of the wafer to be as planar as possible.)
Those skilled in the art will appreciate that a variety of factors may contribute to producing variations across the post-polish surface of a wafer. For example, variations in the surface of the wafer may be attributed to drift of the chemical mechanical polishing device. Typically, a chemical mechanical polishing device is optimized for a particular process, but because of chemical and mechanical changes to the polishing pad during polishing, degradation of process consumables, and other processing factors, the chemical mechanical polishing process may drift from its optimized state.
Generally, within-wafer uniformity variations (i.e., surface non-uniformity) are produced by slight differences in polish rate at various positions on the wafer. FIG. 1 illustrates two radial profiles of surface non-uniformity typically seen after an oxide polish of a wafer. The dished topography is often referred to as a center-fast polishing state because the center of the wafer polishes at a faster rate than the edge of the wafer. The domed topography is designated center-slow because the center of the wafer polishes at a slower rate than the edge of the wafer. For obvious reasons, the dished topography may also be referred to as edge-slow, and the domed topography may also be referred to as edge-fast.
In addition to process drift, pre-polish surface non-uniformity of the wafer may also contribute to producing variations across the post-polish surface of the wafer. For example, prior to being polished, the radial profile of the wafer may exhibit either a domed or dished topology, and the non-uniform polishing characteristics of the polishing tool may exacerbate non-uniformity problem.
One technique for reducing the post-process variability in a polishing tool involves measuring the pre-polish surface profile of the incoming wafers and automatically adjusting the operating recipe of the polishing tool to account for the non-uniformity. One variable controlled in the operating recipe is the arm oscillation length of the polishing tool. An exemplary automatic control technique is described in U.S. patent application Ser. No. 09/372,014, entitled, xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING WITHIN-WAFER UNIFORMITY IN CHEMICAL MECHANICAL POLISHING,xe2x80x9d and currently assigned to the assignee of the present application. The operating recipe of the polishing tool may be controlled on a lot-by-lot or wafer-by-wafer basis.
Adjusting the operating recipe for each incoming lot or wafer may not effectively control the post-polish variability. Typically, an automatic control technique reacts well to relatively small variations in a stable process. Incoming wafers, however, may have widely varying topologies. For example, one lot may have a dished topology, while the next lot has a domed topology. To transition between a dished and a domed topology, the operating recipe may have to be altered significantly. Such widely varying operating parameters reduce the effectiveness of an automatic controller.
Furthermore, a particular polishing tool may have an inherent tendency to polish wafers either center-fast or center-slow. For example, U.S. patent application Ser. No. 09/627,737, entitled, xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING WAFER UNIFORMITY IN A CHEMICAL MECHANICAL POLISHING TOOL USING CARRIER HEAD SIGNATURES,xe2x80x9d and currently assigned to the assignee of the present application, describes a technique that includes grouping carrier heads with similar processing characteristics in a multi-head polishing tool. Such a grouping may increase the likelihood of a center-fast or center-slow tendency in the polishing tool. Attempting to force a polishing tool having an inherent tendency to polish in accordance with one polishing profile to polish using the other type of profile may also reduce the effectiveness of the automatic controller and lead to increased post-polish variability.
There are other types of processing tools that have similar tool state issues. For example, in a plasma etch tool, the plasma power affects the rate of etch in the center relative to that at the edge. Generally, a lower plasma power results in an increased etch rate in the center relative to the edge. The specific relationship between power and etch rate is dependent on factors such as the particular etch tool and the recipe being used. The relationship for a particular configuration may be determined empirically and a mathematical model may be derived. Adjusting the plasma power for each incoming lot of wafers may also result in decreased automatic controller effectiveness issues similar to those described above for a polishing tool.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a method for controlling the flow of wafers through a process flow. The method includes monitoring operating states of a plurality of processing tools adapted to process wafers; measuring a characteristic of a particular incoming wafer; identifying a particular processing tool having an operating state complimentary to the measured characteristic; and routing the particular incoming wafer to the particular processing tool for processing.
Another aspect of the present invention is seen in a manufacturing system including a plurality of processing tools adapted to process wafers and a process control server. The process control server is adapted to access metrology data related to a characteristic of a particular incoming wafer, identify a particular processing tool having an operating state complimentary to the characteristic, and route the particular incoming wafer to the particular processing tool for processing.